The invention relates to a vessel comprising a riser or tendon tensioning construction having at least two spaced apart mounting points and a connector carrying two or more risers or tendons which are with one end attached to the seabed and with the other end attached to the connector, the connector being suspended from the mounting points by at least two suspension members which are movably connected to the mounting points, the suspension members being with a first end attached to respective positions on the connector and with their second end to a respective tensioning member for exerting a tensioning force on the risers or tendons.
From U.S. Pat. No. 4,567,842 a mooring system for a floating production vessel is known comprising a riser which is tensioned by a weight type motion compensating system. Herein the tensioning construction comprises a pivotable frame at the bow of the vessel which at one end is provided with a large counterweight near deck level. The known system has as a disadvantage that it takes up a lot of space and that during roll, pitch or heave movements of the vessel the large mass of the tensioning construction can give rise to an unbalance and exerts large forces on the supporting frame structure.
From U.S. Pat. No. 4,272,059 a riser tensioning system is known wherein a riser, such as a drilling riser, is at its upper end provided with a tension ring which is connected via cables to sheaves on the drilling vessel. The sheaves are mounted on the free ends of piston rods of hydraulic cylinders, the second end of the cables being attached to the vessel. Upon heave, roll or pitch of the vessel, the tensional forces on the riser are maintained generally constant by movement of the piston rods against the hydraulic pressure in the cylinders. This system has as a disadvantage that the tensional forces exerted on the riser will vary with the buoyancy of the vessel. In order to obtain a relatively large stroke of the cylinders the cylinders should be relatively long and therefore take up a lot of space, which in view of the moving nature of the cylinders cannot be effectively used. Furthermore, the hydraulic system is relatively complex.
From U.S. Pat. No. 3,681,928 a barge supporting a drilling rig is known, in which a platform is movably suspended from two mounting arms above deck level of the barge. The platform is connected to the seabed via two parallel cables, or tendons, which pass through openings in the platform and through a central well in the barge. By this construction the platform remains in a horizontal position and at a constant height above the seabed when the vessel moves vertically due to wave motion. Under the influence of the dependent counterweights, the cables are kept taut. This construction has as a disadvantage that upon movement of the barge relatively large inertia forces may be exerted on the sheaves by the counter weights, and that large forces are exerted on the cables by the swinging counterweights. Furthermore, the freely swinging counterweights may form an obstruction for personnel on deck of the drilling barge, and take up a lot of space as they should be clear from any structural parts of the barge, especially in situations of high seas.
It is therefore an object of the present invention to provide a riser and/or tendon tensioning construction which can be used in deep waters using a dry production tree, which consumes relatively little space and which is stable under different motions of the vessel. It is a further object of the present invention to provide a riser tensioning construction which allows attachment of multiple risers while maintaining a substantially equalised tensional force on the risers upon movements of the vessel. It is a further object of the present invention to provide a riser tensioning system which can also function as a stable support platform for production or drilling equipment. It is another object of the present invention to provide a tensioning construction which can be used in deep waters to support a metal pipe, or riser, passing from a sub sea structure to a deck supported on a floating vessel. The risers may convey hydrocarbon well production fluids to production trees on the riser supporting deck, or alternatively be used to convey flow between the sub sea structure and the deck. The upper and lower connections of the risers may be ridgid with bending taking by the pipe, or may include pivoting means.
Thereto the riser tensioning construction according to the present invention is characterised in that the tensioning member extends outside the hull of the vessel or through a well in the vessel such as to be located below water level.
By placing the counterweight below water level, the forces exerted by the counterweight on the vessel upon movement thereof are reduced and the motions of the counterweight are damped. Furthermore, location below water level of the counterweight provides for an easy way of varying the tensional force exerted thereby, not only by varying the mass thereof but also by varying the buoyancy.
The riser and/or tendon tensioning construction according to the present invention is particularly useful in deep waters as it allows rigid risers to pass from greath depths to the surface, using only proven components that can take up considerable forces and external pressures. Pipes and flowthrough pivot joints are available for these pressures.
The suspension member may be a cable that is guided along a sheave, but is preferably formed by a pivoting arm, which is less subject to wear compared to a cable-sheave system. The tensioning member according to the present invention may be formed by a counterweight either directly attached to one free end of the pivot arm, or attached to the pivot arm via a cable. The tensioning member may comprise a cable that is attached to the seabed by anchoring means such as a clump weight, a suction anchor or a pile, for exerting a tensioning force on the risers and/or tendons, in which case it is preferred that the cable is elastic, such as for instance a polyester cable. It is furthermore possible that the suspension member and the tensioning member are formed by a single cable which continues along the cable guide means to extend towards the seabed.
It is noted that from WO 98/18673 a mooring system is known in which a cable extends from the seabed towards deck level of the vessel to be directed around a sheave back to a counterweight freely suspended from the cable below sea level, for the reduction of mooring loads attributable to oscillating wave drifts. The tensioning system described therein uses for each mooring line a separate counterweight and is not flexible in case several risers or anchor lines need to be added to the vessel.
In a further embodiment according to the present invention each suspension member is with its first end attached to the connector on one side of a centre line of the vessel, the mounting point of the respective suspension member being located on the other side of the centre line. In case the tensioning member comprises counterweights located above or below water level, placing the weights on the opposite side of the ship with respect to the point in which the suspension member is attached to the connector, an angular compensation for the roll and pitch motions is achieved, which results in little to substantially zero vertical movements of the hanging weights.
In a further embodiment according to the present invention the mounting points comprise at least two spaced apart mounting arms each carrying a cable guide means and a respective cable, the connector being supported by the first ends of the cables, preferably above deck level. In this embodiment the motions of the vessel are completely decoupled from the risers. A substantially constant tensional force is exerted on the risers and/or tedons upon heave, pitch or roll of the vessel. As the mounting arms according to the present invention remain stationary, they do not form an obstruction for the drilling and production equipment on the vessel.
The connector can for instance be formed by a support arm extending between the mounting arms in the length or width direction of the vessel. The support arm, preferably supporting multiple risers, is lowered or raised a small amount that is determined by the elasticity of the risers, at the respective side at which the tension in the riser increases or decreases by lifting or lowering of the counterweights. Besides dynamic forces acting on the counterweights and frictional forces in the cable guide means, the tensional forces on the risers remain substantially constant and are substantially independent of the movements of the vessel. Furthermore, the support arm can be effectively used as, or be part of a stable deck structure for supporting drilling or production equipment, as it will be maintained in a substantially horizontal position by the tensional forces of the risers acting therein.
The riser tensioning construction according to the present invention may be mounted on a turret structure of a vessel around which the vessel can weathervane, at deck level or at keel level thereof. It is also possible to use the present riser tensioning construction in a vessel wherein the cables and counterweights extend in a central well, for instance through the turret.
To prevent lateral motions of the tensioning weight it is possible to provide a weight guiding element on the vessel, for instance near keel level or near the seabed. It is also possible to guide the tensioning weights along the risers, in case a rigid steel casing is used.